Nuclear control systems for pressurized water reactors are known which are based on average reactor coolant temperature as a set point. These known control systems utilize either a constant average temperature set point or a set point which increases with increasing reactor power. Control of the average temperature to the set point is by reactor control rod movement and/or changes in the boron concentration in the reactor coolant. Rapid power changes have historically depended upon full length control rod motion for reactivity addition and upon partial length control rod motion for power distribution control within the reactor. However, the large changes in linear heat rate experienced by reactor fuel rods in the vicinity of the control rods, particularly those near the partial length control rods, have led to recent concerns about fuel cladding integrity in light of a phenomenon known as pellet-clad interaction.
It is well-known that reactor operation with the partial length control rods totally removed from the reactor reduces the risk for pellet-clad interaction but at the same time restricts the maneuvering capability of the reactor based on full length control rods because there is no readily available method of controlling large power imbalances within the core. Rapid power changes utilizing changes in boron concentration, while possible would require very expensive hardware modifications to existing reactor designs and would substantially increase the radioactive waste processing requirements.
Because the reactor coolant has an inherent negative moderator coefficient, methods of achieving positive reactivity addition and fast power increases by decreasing the average reactor coolant temperature are known. However, all the known methods employ a temperature drop below a set point which normally increases with power which is the standard control mode for nuclear plants with recirculating steam generators.
What was needed was a control system which would allow large power load changes at rates up to five percent full power per minute and which could be implemented at a reasonable cost in conjunction with deletion of the partial length control rods from the existing design of a nuclear reactor system utilizing once-through steam generators.